Shear's Cysts of the Oral and Maxillofacial Regions. Paul M. SpeightЧитать онлайн книгу.
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In the majority of odontogenic cysts, the epithelial lining is derived from epithelial remnants of the dental lamina (Table 2.1). Early in the development of the jaws, the surface epithelium thickens and grows downwards into the mesenchyme of the future dental arches to form the dental lamina. This extends around the arch as a band that maps the future sites of tooth bud formation for both primary and secondary dentitions. The teeth develop as a result of complex epithelial–mesenchymal interactions that result in epithelial thickenings or placodes, which then form the enamel organs that pass through the well‐described bud, cap, and bell stages during formation of the fully developed tooth (Nanci 2017 ). The dental lamina remains as a thin band that joins the surface oral epithelium to the enamel organ and only disintegrates at the late bell stage of tooth development. Disintegration of the dental lamina results in the formation of small epithelial islands that lie over unerupted teeth, but also remain in the tissues adjacent to the teeth after eruption. The disintegrating vdental lamina is illustrated in Figure 9.9. The epithelial cell rests of the dental lamina give rise to most of the odontogenic cysts (Table 2.1) as well as to most odontogenic tumours. Dental lamina rests are particularly numerous at the posterior aspect of the dental arches and in the tissues overlying unerupted teeth and in the dental follicle. This accounts for the fact that the angle of the mandible is a common site for many cyst types, and that many cysts (and tumours) may arise in the dental follicle and embrace or surround an unerupted tooth and lie in a dentigerous relationship. This especially affects the mandibular third molars, since these are the most commonly impacted teeth (Brown et al. 1982 ).
Although most types of odontogenic cyst arise from dental lamina, the most common cyst (radicular cyst) takes its origin from the rest cells of Malassez that lie in the periodontal ligament as remnants of Hertwig's root sheath (see Figure 3.6). The second most common cyst, the dentigerous cyst, arises from the reduced enamel epithelium that embraces the fully formed crown of a tooth prior to eruption. In the case of the radicular cyst, the phases of cyst formation and growth are well understood and are driven by inflammation that is initiated by bacterial factors emanating from a non‐vital pulp. This process is described in detail in Chapter 3. In developmental cysts, however, the processes are less clear, but are almost certainly driven by epithelial–mesenchymal interactions that initiate the molecular signalling pathways that underpin normal tooth development, morphogenesis, and eruption. Thus, the mechanisms of formation of developmental cysts can be regarded as the aberrant expression of normal processes.
Normal tooth eruption provides a good model of the epithelial–mesenchymal interactions that regulate tissue remodelling, including cell proliferation and bone resorption, and involve complex cascades and networks of cytokines, chemokines, and growth factors (Wise et al. 2002 ; Nel et al. 2015 ; Bastos et al. 2021 ). These biological factors are discussed in detail in the context of the radicular cyst (Table 3.2), where the cascade is started by bacterial infection, but the same factors are involved in normal development and tooth eruption and contribute to the pathogenesis of developmental cysts. For example, Il‐1α is a pivotal pro‐inflammatory cytokine that can be activated by bacterial endotoxins, but it also has an important role as a paracrine signalling molecule in the dental follicle where, along with other factors (e.g. parathyroid hormone–related protein) it mediates tissue remodelling and bone resorption during tooth eruption via its role as an activator of osteoclasts.
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